The two men talking in the Carousel Club on Commerce Street had a shared problem: US attorney general Bobby Kennedy. It was October 4 1963 in Dallas, Texas, and Lee Harvey Oswald and Jack Ruby had a plan.

"There is a way to get rid of him without killing him," Oswald, an angry, idealistic young man tells Ruby, a club owner. "I can shoot his brother."

"But that wouldn't be patriotic," Ruby warns.

"I can still do it," Oswald tells him. "All I need is my rifle and a tall building; but it will take time, maybe six months to find the right place; but I'll have to have some money to live on while I do the planning."

The transcript of a conversation between Ruby and Oswald, less than two months before the assassination of President John F Kennedy, would be one of the most sensational revelations about an event that has fuelled conspiracy theories for more than 40 years. But there is one problem with the document unearthed in a safe on the 12th floor of the Dallas courthouse: it is almost certainly a fake.

The document was discovered along with a holster that probably held the gun Ruby used to shoot Oswald, clothing belonging to the two men, and hundreds of letters sent to Ruby by admirers and critics following his shooting of Oswald. The safe also held a movie contract for $1m signed by former district attorney Henry Wade, the prosecutor in the trial of Ruby.

"The fact that it's sitting in Henry Wade's file, and he didn't do anything, indicates he thought it wasn't worth anything," Gary Mack, the curator of a museum dedicated to the Kennedy assassination, told the Dallas Morning News. "He probably kept it because it was funny. It's hilarious. It's like a bad B-movie."

More than a B-movie, Wade planned a semi-documentary treatment of the assassination, with the working title Countdown in Dallas. The supposed conversation between Ruby and Oswald, which took place on a night when Oswald is known to have been with his wife, is thought to have been part of the film script.

The existence of the cache of about a dozen boxes was made public by Dallas county district attorney Craig Watkins. He said every Dallas district attorney since the assassination had known about it, but all had kept it secret.

It also held the contents of Ruby's pockets on the night he was arrested: two sets of brass knuckle dusters and a holster.

Ruby's conviction for killing Oswald was overturned on appeal and he died of cancer in 1967 before a second trial.